allow init and initWithConfig work with session mgr
Fixing behavior if response is `nil`

Testing without internet connection, `operation.response` is `nil` and the `failure` block is not called. We can get the URL from the `operation.request` instead of `operation.response` (as done in the `success` block).
Replacing string literals in NSCoding with NSStringFromSelector(@selector()) pattern
aCoder / aDecoder -> coder / decoder
[Issue #1517] Fix implementation of batchOfRequestOperations:progressBlock:completionBlock: for nil when passed empty operations parameter
Merge pull request #1505 from marcelofabri/patch-1

[2.0] Fixing behavior if response is `nil`
Merge pull request #1503 from subdigital/session-manager-initializers

Allow init and initWithSessionConfiguration: to work with AFHTTPSessionManager